Oxides of nitrogen, or NOx, is one of the components in internal combustion engine emissions. A common method for reducing NOx is through the recirculation of a fraction of engine exhaust gases back into the air inlet of the engine to be combined with the incoming air charge. This process is often called charge dilution or exhaust gas recirculation (EGR). By introducing a combination of fresh inlet air and exhaust gases into the engine, the heat absorbing capacity of the air charge is increased and the overall oxygen content of the air charge is decreased. Increasing the heat absorbing capacity of the air charge suppresses or reduces engine combustion temperature, thereby inhibiting NOx formation. Decreasing the oxygen content of the air charge decreases NOx formation by reducing the availability of one of its constituent elements.
EGR typically involves recirculation of exhaust gases through an EGR passage between an engine exhaust conduit and an engine fresh air intake passage. A valve within the EGR passage, the EGR valve, is controlled to vary a restriction within the EGR passage to regulate the flow of exhaust gases therethrough. When EGR is not required, the EGR valve is driven to a full restriction (closed) position. When EGR is required, the EGR valve is driven to an open position through application of a position control signal to the EGR valve. The degree of opening of the EGR valve varies with the magnitude of the position control signal.
Typically, the exhaust gases within the EGR passage are cooled, prior to mixing the exhaust gases with the fresh inlet air, by passing the exhaust gases through a heat exchanger. A typical heat exchanger for this application will facilitate the transfer of heat energy from the exhaust gases to a liquid cooling medium, such as the engine coolant. This type of heat exchanger is commonly referred to as an air-to-water heat exchanger.